Add 'flatpak' to the set of packages for the ostree-installer runroot
Use a newly added Pungi option (https://pagure.io/pungi/pull-request/1278),
so that flatpak is in the Silverblue ostree-installer runroot, which is
needed for embedding Flatpaks in the Silverblue image.
Signed-off-by: Owen W. Taylor <otaylor@fishsoup.net>